Sewing machines have conventionally allowed execution of a utility sewing operation and a so called free motion sewing operation. In the utility sewing operation, a feed dog provided inside a sewing machine bed is moved back and forth to feed a workpiece cloth. In the free motion sewing operation, on the other hand, a user is allowed to manually move the workpiece cloth freely while keeping the feed dog lowered below a needle plate provided on the upper surface of the sewing machine bed. One of the disadvantages of the free motion sewing operation is that it produces poor looking stitches when the stitch pitch is inconsistent and misaligned. Formation of neat looking stitches with consistent or uniform stitch pitch requires a high degree of technical maturity on the part of the user.
JP 2002-292175 A discloses a sewing machine having an arm provided with a needle bar and a downwardly-oriented image sensor for capturing images of sewing operation. The needle bar extends downward from the extreme end of the arm and has sewing needle attached to it. The disclosed sewing machine executes free motion sewing operation by allowing a user to freely move a workpiece cloth placed on the upper surface of the sewing machine bed in manually adjusted feed amounts. Some of the images captured by the image sensor during the free motion sewing operation are taken into a microcomputer provided in the sewing machine as static images at predetermined time interval. By comparing the latest static image with the static image immediately preceding it, the microcomputer obtains the feed amount of the workpiece cloth based upon which the rotational speed of a sewing machine motor is altered; which is another way of saying that the speed of vertical movement of the needle bar is altered. Since the vertical movement of the needle bar is decelerated when there is relatively less feed amount and accelerated when there is relatively greater feed amount, even a user inexperienced in free motion sewing operation can form stitches in uniform stitch pitch.
According to the above configured sewing machine, since the speed of the needle bar movement, in other words, sewing speed of the free motion sewing operation is automatically changed depending on the feed amount of the workpiece cloth, sewing speed may be accelerated in greater degree than intended depending on the feed amount.